1. Field of the Invention
The invention relates to a process for fabricating non-volatile memory by tilt-angle ion implantation, and in particular, to a process for fabricating non-volatile memory by tilt-angle ion implantation characterized in that, in addition to increase the retention time of the memory, it can avoid merge of electrons due to prolonged operation.
2. Description of the Prior Art
Recently, the role of the flash memory device in the development and fabrication of the semiconductor device has becomes more and more important. As portable electronic products increasingly prevail, the modification and portability of data has a pivotal function in these products, and accordingly, a flash memory has its competitive advantage over other devices.
Referring to FIGS. 1 and 2, a schematic structure of a conventional non-volatile memory is shown to be composed of two different structures: a floating gate device 1 (FIG. 1) and a charge trapping device 2 (FIG. 2).
As shown in FIG. 1, the manner of memory storage of the floating gate device 1 consists of storing electrons on a floating gate 11 made of polysilicon. Polysilicon is a good electron conducting material such that, after capturing electrons on the floating gate 11, electrons can distribute uniformly thereon, and as a result, the shift of the threshold voltage thus generated can be used for determining the action of memory. However, the floating gate device 1 can store only one bit so that its production cost is correspondingly increased and hence exhibit substantially no economical benefit.
Referring to FIG. 2, the charge-trapping device 2 is composed of two or three layers of insulting materials stacked beneath the gate G, and wherein one of these layers has a high deep-level trap density, which is an insulating material capable of capturing more electrons. This material includes silicon nitride 22, Si3N4, or alumina, Al2O3, which can store charge. Unfortunately, the charge trapping device 2 shows a big problem on retention time, because oxides 21 in the underlying layer might capture positive charge therein during programming that may lower the energy barrier of the underlying oxide layer 21 and as a result, electrons captured in the silicon nitride 22 might penetrate readily through the underlying oxide layer 21. This would cause loss of electrons stored therein and consequently reduces the retention time. Further, electrons captured in both ends of silicon nitride 22 will cumulate and bind together gradually as the operation time increases. Under this circumstance, the original meaning of two-bit storage will be deviated naturally. This mechanism is the biggest problem encountered in the current 2-bit storage.
Accordingly, it is apparent that the conventional device has many disadvantages and is not a perfect design such that it is desirable to be improved.
In view of the above-described disadvantages associated with the prior art non-volatile memory, the inventor has devoted to improve and innovate, and, after studying intensively for many years, developed successfully the process for fabricating non-volatile memory by tilt-angle ion implantation according to the invention.